Recording materials used in the heat-sensitive recording process include a leuco coloring type heat-sensitive recording material which is commonly used. However, such a heat-sensitive recording material is disadvantageous in that it is subject to undesirable decoloring or color development when handled roughly after recording or brought into contact with an adhesive tape or diazo copying paper.
In order to develop a heat-sensitive recording material free from such a defect, diazo coloring type heat-sensitive recording materials have been intensively studied in recent years. However, diazo compounds are inherently unstable and, therefore, often develop colors on exposed portions thereof under various conditions.
A process is known which comprises thermally recording letters or images on a light- and heat-sensitive recording material comprislng a diazo compound, a coupling component and an alkali producing agent or a color development assistant, and then irradiating the recording material with light to decompose the unreacted diazo compound so that the color development is interrupted, as disclosed in Japanese patent application (OPI) Nos. 123089/82 and 125092/82 (corresponding to U.S. Pat. No. 4,411,979) (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"). However, such a recording material is disadvantageous in that it is subject to gradual precoupling during storage which may cause undesirable coloring (fog). In this respect, Japanese patent application (OPI) No. 190886/84 proposes incorporating at least one of a diazo compound, a coupling component and a color development assistant into the core of a microcapsule.
A light-fixable light-sensitive recording material utilizing such a microcapsule is advantageous in that it needs only a simple recording apparatus and is excellent in shelf life stability and stability of images and background after recording.
However, the heat-sensitive recording material utilizing the permeability of the wall of such a microcapsule caused by heating is disadvantageous in that when water attaches to the recording layer, the heat-sensitive layer may be peeled off, or when its recording material is printed by a thermal head, sticking, stains in the thermal head or other troubles may be caused. In order to improve such a heat-sensitive recording material, a protective layer is provided on the heat-sensitive layer.
Examples of such a heat-sensitive recording paper with an outer protective layer having improved water resistance and exhibiting neither sticking nor stains in the thermal head include those containing a hydrophobic high molecular weight compound or various water-soluble high molecular weight compound and optionally containing a waterproofing agent or a filler as proposed in Japanese Patent Publication No. 27880/69 (corresponding to U.S. Pat. No. 3,445,261) and Japanese patent application (OPI) Nos. 30437/73, 31958/73, 19840/78, 14751/79, 53545/79, 111837/79, 128349/79, 126193/81, 139993/81, 10530/82, (corresponding to U.S. Pat. No. 4,346,343), 29491/82, 1053925/82, 115391/82 (corresponding to U.S. Pat. No. 4,415,627), 144793/82 (corresponding to U.S. Pat. Nos. 4,444,819 and 4,551,738), 107884/82, 53484/83 and 193189/83.
However, these approaches are disadvantageous in that the water resistance of the recording material thus obtained is still insufficient, such exhibit low sensitivity, the antisticking property of the recording material thus obtained is insufficient, the recording material thus obtained develops blocking, and such require a complicated production process, resulting in high production costs. Thus, in general, these approaches are not industrially practical.